1. Field of the Invention
The present invention relates to a process for preparing esters of acrylic acid or methacrylic acid [(meth)acrylic acid].
2. Discussion of the Background
(Meth)acrylic esters are generally prepared industrially by esterification of (meth)acrylic acid with alkanols in the presence of strong acids as esterification catalysts (e.g. a mineral acid such as sulfuric acid, phosphoric acid, alkanesulfonic acids or arylsulfonic acids). Such processes are known from, for example, Kirk Othmer, "Encyclopedia of Chemical Technology", Vol. 1, pp. 347-348. The catalyst content of the esterification mixture can vary from tenths of a percent to several percent. When using polybasic mineral acids as catalyst, the mineral acid is easily esterified by the alkanol present to form the monoester which is the actual esterification catalyst. After the esterification is complete, the reaction mixture contains a relatively large amount of this monoester.
The acids used as catalysts and any esters thereof which are formed have to be removed from the reaction mixture before further processing. This is generally achieved by scrubbing and neutralizing the reaction mixture with alkali metal and alkaline earth metal hydroxide solution or carbonate solutions. This results in the formation of wastewater whose disposal is costly and environmentally polluting. If sulfuric acid is used as catalyst, then, as mentioned, the monoester of sulfuric acid with the alkanol concerned is predominantly formed. The salts of the sulfuric acid monoesters, in particular the esters of higher alkanols, are surface-active and on disposal would considerably impair the quality of the wastewater from the process and cause a not inconsiderable loss of useful product. Thus, for economic and ecological reasons, the recovery and reuse of the catalyst is desirable.
The prior art discloses a number of processes which are all, however, encumbered by considerable disadvantages.
EP-A-0 609 127 describes a process for preparing (meth)acrylic esters, with the alcohol component being recovered by acid hydrolysis from the corresponding sulfuric monoester which is formed in the esterification from sulfuric acid and the alcohol. This process is complicated, environmentally polluting and uneconomical.
CZ-B-179 808 describes a process for recovering mineral acids from esterification mixtures by extraction of the esterification mixture with water, concentration of the aqueous phase by distillation and recirculation of the concentrated aqueous catalyst solution thus obtained to the esterification reaction. This process is energy-consuming.
EP-A-0 618 187 (= U.S. Pat. No. 5,386,052) describes a process for preparing (meth)acrylic esters in which the catalyst is extracted with water and the extract is, possibly after concentration by distillation, returned to the esterification reaction. However, this document draws particular attention to the fact that sulfuric acid is unsuitable as catalyst owing to the poor extractability of the monoalkyl sulfate because the large amount of water which would be needed for adequate extraction of the monoalkyl sulfate would adversely affect the esterification reaction. Catalysts used are therefore alkylsulfonic or arylsulfonic acids (column 2, lines 55 ff), but these are considerably more expensive than sulfuric acid.
In all known processes, the esterification of the starting materials and the distillative removal of the water of reaction are carried out in one and the same reaction vessel or in a cascade of similar vessels. Since the distillation capacity of the reactor can only be utilized sufficiently when relatively large amounts of water have been formed by the esterification reaction, the conventional processes are not optimal in terms of the space-time yield and economics.
Increasing the conversion by raising the reaction temperature is possible to only a limited extent because of the polymerization tendency of the acrylic compounds.